However, this practice occurs on many flights. While flying SQ from US/West Coast to India dozens of times... the SFO/LAX - SIN flight would stop in HKG or ICN or NRT ... and all passengers had to get off the plane, go through security, and re-board the same plane! ( Never understood why! )

However, I flew Cathay from Vancouver - HKG once. That flight arrived into Vancouver from JFK (If I am not mistaken), and the continuing passengers did NOT de-plane. They got to sit inside.

So, I have experienced both ways....I am not sure who determines the practice - the airline, or the airport.

If AI 126 requires deplaning & security check that would not surprise me (although, the twist here is, it is a domestic stop as opposed to an international stop).

Although I do not know about specifics of AI126, there are usually valid reasons for that:

-often on tag ons airlines perform a crew change. That the passengers can't sit on a plane without a crew in there goes without saying. Thus passenger need to deboard. This may involve an additional security screening if the local reregulations require this

-In some instances - i.e. when a stop occurs in the US - local regulation requires passenger to perform immigration. Maybe some countries also require passengers to deboard to repass local security, alhtough not need to pass immigration.

-Sometimes, and this may be the case with AI126 and may be true also for Saudi Arabia (although not sure), planes come in internationally, and have a second domestic leg. In some instances airlines will sell seats for the domestic leg to domestic travellers, thus the plane will arrive as a domestic flight at the final destination not requiring passengers to pass immigration. In this case passengers need to deplane and pass immigration in the first point of entry of that country.

Quoting DTW2HYD (Thread starter):Someone told me ORD-HYD passengers on AI126 have to deplane at DEL for security check and board same plane again. Is this true? Any reason why AI would do this?

I was under the impression that HYD passenger continue to sit on the plane while ORD-DEL passengers deplane.

It's an almost universal rule that you have to deplane and pass through customs/immigration at your first point of entry in the country. This is especially true if the continuing flight is domestic and local passengers will be allowed - if there are DEL-HYD local pax boarding your flight, how would they separate those who do and do not need to pass through the formalities on arrival at HYD? By handling that all at DEL, the DEL-HYD flight can be handled as a typical domestic arrival.

Quoting steex (Reply 3):It's an almost universal rule that you have to deplane and pass through customs/immigration at your first point of entry in the country. This is especially true if the continuing flight is domestic and local passengers will be allowed - if there are DEL-HYD local pax boarding your flight, how would they separate those who do and do not need to pass through the formalities on arrival at HYD? By handling that all at DEL, the DEL-HYD flight can be handled as a typical domestic arrival.

That the interesting part. Immigration and Customs for ORD-HYD passengers is at HYD.

What I read on some other site DEL-HYD passengers go through immigration, their domestic boarding pass gets immigration stamp, they don't need passport. They cannot buy duty free without passport. In HYD their boarding pass again gets stamped by immigration and they walk thru green channel. It is mind boggling, but anything is fair game at Indian airports.

That is really a strange process Reminded me of Riga airport when Lithuania were not part of Schengen and thus every flight was an international one and needed immigration for departures/arrivals. The country is so small that they do not have domestic flights, until the day that Air Baltic started flights to Liepaja on the Baltic Coast.

Too bad that the airport at that time had no domestic gates, and thus you had to pass passport control, although they normally did not throughly checked your passport. These domestic flights costed less than 10 EUR incl tax, and in theory people could leave Lithuania illegally by buying such a ticket, using the domestic pass to pass passport control (which didn't cared since you were on that odd flight to Liepaja) but then taking another international flight with another boarding pass.

Quoting steex (Reply 3):It's an almost universal rule that you have to deplane and pass through customs/immigration at your first point of entry in the country

I would disagree with "almost universal" part of your comment for aircraft continuing on within a country. It is true in some countries, not in others.

In Australia you always go thru C&I at your LAST port of call on an International flight. Example: QF8 DFW-BNE-SYD. If your destination is BNE or you are making a domestic connection in BNE then you do C&I in BNE. If your destination or domestic connection is in SYD you do C&I in SYD. Domestic pax are carried BNE-SYD & the flight operates between International terminals. Domestic pax require photo id and are issued a "D" (for domestic) boarding pass.
Another example is UA at SYD. Two B744s arrive about 0700, from LAX & SFO. One is parked for a few hours, while the other flies the SYD-MEL-SYD tag. Pax from LAX/SFO to MEL do C&I in MEL. pax from MEL to LAX/SFO do outwards C&I in MEL. UA of course does not carry domestic pax SYD-MEL-SYD.

It's really difficult to generalise about this as each country is different.

Quoting DTW2HYD (Reply 4):What I read on some other site DEL-HYD passengers go through immigration, their domestic boarding pass gets immigration stamp, they don't need passport.

That's exactly what it was like for me between FRA-DEL and DEL-MAA, both on AI. You get the full monty in Delhi: hand over int'l boarding pass at a transfer desk, have "T" written on your BP for the onward flight. Go through security and don't forget to put extra tags on your hand luggage so they can stamp them after you've passed the checkpoint. Fly to your destination - from the int'l terminal - and have your passport stamped there.

Quoting jkrugman22 (Reply 7):Isn't there two separate airports in DEL (a la BOM)? One for domestic flights and one for Intl?

Quoting reifel (Reply 2):That the passengers can't sit on a plane without a crew in there goes without saying. Thus passenger need to deboard. This may involve an additional security screening if the local reregulations require this

In the States, crew changes with passengers aboard happen on a daily basis, particularly on WN. Sometimes, airlines will use the 'old' crew to board the 'new' flight if the 'new' crew is late. There's no regulatory issue with that here as long as someone is aboard at all times.

Quoting reifel (Reply 2):often on tag ons airlines perform a crew change. That the passengers can't sit on a plane without a crew in there goes without saying

The old crew will wait for the new crew to board.....this is not the reason.

Quoting steex (Reply 3):if there are DEL-HYD local pax boarding your flight, how would they separate those who do and do not need to pass through the formalities on arrival at HYD? By handling that all at DEL, the DEL-HYD flight can be handled as a typical domestic arrival.